git-gui: Change repository browser radio buttons to hyperlinks

git-gui: Change repository browser radio buttons to hyperlinks

Making a user click twice to select which action they want to perform
when starting git-gui is just wasting their time.  Clicking once on a
radio button and then clicking again on the "Next >" button is quite
unnecessary.

Since the recent repository list is shown as a list of hyperlinks we
now offer the 3 basic startup actions as hyperlinks.  Clicking on a
link will immediately jump to the next UI panel, saving the user time
as they don't need to click an additional button.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>